The present invention relates to a foamable silicone rubber composition and a foamed silicone rubber body from the composition. More particularly, the invention relates to a foamable silicone rubber composition capable of giving a cured and foamed silicone rubber body having a controlled low rate of recovery from elastic compression so as to return to the uncompressed state taking a length of time and to be useful as a material of earplugs for sound deadening or swimming as well as vibration- and impact-absorbing materials, sound-insulating materials, medical pads, cushions and the like used in industries, medical treatments, sporting goods and elsewhere.
Typical foamable silicone rubber compositions known in the prior art are prepared by compounding a silicone rubber composition comprising an organopolysiloxane and a finely divided silica filler with an organic blowing agent, such as azobisisobutyronitrile, azodicarbonamide and the like, and an organic peroxide as a curing agent. Such a foamable silicone rubber composition can be foamed and cured by heating to give a foamed silicone rubber body. The foamed silicone rubber bodies thus obtained are usually excellent as an elastic material and, when the body under compression is released from the compressive force, the body immediately regains the uncompressed form within only a few seconds. Although such a rapid elastic recovery from the compressed state is not always disadvantageous, foamed silicone rubber bodies are sometimes desired to show a slow rate of elastic recovery from the compressed state, for example, in the above mentioned applications.
Known foamable silicone rubber compositions of another class are formulated with a vinyl-containing organopolysiloxane having vinyl groups bonded to the silicon atoms, an organohydrogenpolysiloxane having hydrogen atoms directly bonded to the silicon atoms, a platinum-based catalytic compound and a thermally decomposable organic blowing agent which produces a gaseous decomposition product (see, for example, Japanese Patent Publication No. 45-12675). A problem in the foamable silicone rubber compositions of this class is that the expansion of the composition by foaming is limited not to give a foamed rubber body of high expansion as desired.
Foamable silicone rubber compositions of a further known class utilize the dehydrogenation condensation reaction between a polyhydroxy organopolysiloxane and an organohydrogenpolysiloxane as mentioned above in the presence of a catalyst which may be an organic acid salt of a heavy metal or a quaternary ammonium salt (see, for example, Japanese Patent Publication No. 44-8875). A problem in the foamed silicone rubber body obtained from these compositions is that the body is sometimes too rigid with little elastic resilience in addition to the disadvantage that the organopolysiloxane is subject to depolymerization or degradation when it is brought into contact with water or steam at high temperatures to cause a great loss in the properties as a rubber so that the application fields of these compositions are rather limited.
The inventors have previously discovered that a foamable silicone rubber composition comprising a vinyl-containing dimethylpolysiloxane having vinyl groups bonded to the silicon atoms, an organohydrogenpolysiloxane having hydrogen atoms directly bonded to the silicon atoms, a thermally decomposable organic blowing agent and an organic peroxide can give a foamed silicone rubber body having a controlled rate of elastic recovery from a compressed state by heating to cause foaming and curing when the composition is further admixed with a copolymer of boric acid and a dimethylpolysiloxane terminated at the molecular chain ends with methoxy groups. Though quite effective for controlling the retarded rate of elastic recovery at least immediately after preparation of the foamed body, the effect is gradually lost when the foamed rubber body is stored for a length of time of, for example, three months or longer or when the foamed rubber body is soaked with water so that the foamed rubber body under compression rapidly regains the uncompressed state by releasing the compressive force. Therefore, it is eagerly desired to develop a foamable silicone rubber composition capable of giving a foamed silicone rubber body having a permanently controlled retarded rate of elastic recovery from compression.